1. Field of the Invention
The present invention relates generally to rotary bits for drilling subterranean formations and, more specifically, to superabrasive cutting elements or cutters suitable for use on such bits, particularly of the so-called fixed-cutter or xe2x80x9cdragxe2x80x9d bit variety.
2. Background of Related Art
Fixed-cutter or drag bits have been employed in subterranean drilling for many decades, and various sizes, shapes and patterns of natural and synthetic diamonds have been used on drag bit crowns as cutting elements. Polycrystalline diamond compact (PDC) cutters comprised of a diamond table formed under ultra-hightemperature, ultrahigh-pressure conditions onto a substrate, typically of cemented tungsten carbide (WC), were introduced about twenty-five years ago. PDC cutters, with their diamond tables providing a relatively large, two-dimensional cutting face (usually of circular, semicircular or tombstone shape, although other configurations are known), have provided drag bit designers with a wide variety of potential cutter deployments and orientations, crown configurations, nozzle placements and other design alternatives not previously possible with the smaller natural diamond and polyhedral, unbacked synthetic diamonds previously employed in drag bits. The PDC cutters have, with various bit designs, achieved outstanding advances in drilling efficiency and rate of penetration (ROP) when employed in soft to medium hardness formations, and the larger cutting face dimensions and attendant greater extension or xe2x80x9cexposurexe2x80x9d above the bit crown have afforded the opportunity for greatly improved bit hydraulics for cutter lubrication and cooling and formation debris removal. The same type and magnitude of advances in drag bit design in terms of cutter robustness and longevity, particularly for drilling rock of medium to high compressive strength, has, unfortunately, not been realized to a desired degree.
State of the art substrate-supported PDC cutters have demonstrated a notable susceptibility to spalling and fracture of the PDC diamond layer or table when subjected to the severe downhole environment attendant to drilling rock formations of moderate to high compressive strength, on the order of nine to twelve kpsi and above, unconfined. Engagement of such formations by the PDC cutters occurs under high weight on bit (WOB) required to drill such formations and high impact loads from torque oscillations. These conditions are aggravated by the periodic high loading and unloading of the cutting elements as the bit impacts against the unforgiving surface of the formation due to drill string flex, bounce and oscillation, bit whirl and wobble, and varying WOB. Thus, high compressive strength rock, or softer formations containing stringers of a different, higher compressive strength, may produce severe damage to, if not catastrophic failure of, the PDC diamond tables. Furthermore, bits are subjected to severe vibration and shock loads induced by movement during drilling between rock of different compressive strengths, for example, when the bit abruptly encounters a moderately hard strata after drilling through soft rock.
Severe damage to even a single cutter on a PDC cutter-laden bit crown can drastically reduce efficiency of the bit. If there is more than one cutter at the radial location of a failed cutter, failure of one may soon cause the others to be overstressed and to fail in a xe2x80x9cdominoxe2x80x9d effect. As even relatively minor damage may quickly accelerate the degradation of the PDC cutters, many drilling operators lack confidence in PDC cutter drag bits for hard and stringer-laden formations.
It has been recognized in the art that the sharp, typically 90xc2x0 edge of an unworn, conventional PDC cutter element is especially susceptible to damage during its initial engagement with a hard formation, particularly if that engagement includes even a relatively minor impact. It has also been recognized that pre-beveling or pre-chamfering of the PDC diamond table cutting edge provides some degree of protection against cutter damage during initial engagement with the formation, the PDC cutters being demonstrably less susceptible to damage after a wear flat has begun to form on the diamond table and substrate.
U.S. Pat. Nos. Re 32,036, 4,109,737, 4,987,800, and 5,016,718 disclose and illustrate beveled or chamfered PDC cutting elements, as well as alternative modifications such as rounded (radiused) edges and perforated edges which fracture into a chamfer-like configuration. U.S. Pat. No. 5,437,343, assigned to the assignee of the present application and incorporated herein by this reference, discloses and illustrates a multiple-chamfer PDC diamond table edge configuration which, under some conditions, exhibits even greater resistance to impact-induced cutter damage. U.S. Pat. No. 5,706,906, assigned to the assignee of the present application and incorporated herein by this reference, discloses and illustrates PDC cutters employing a relatively thick diamond table and a very large chamfer, or so-called xe2x80x9crake landxe2x80x9d, at the diamond table periphery.
However, even with the PDC cutting element edge configuration modifications employed in the art, cutter damage remains an all too frequent occurrence when drilling formations of moderate to high compressive strengths and stringer-laden formations.
Another approach to enhancing the robustness of PDC cutters has been the use of variously configured boundaries or xe2x80x9cinterfacesxe2x80x9d between the diamond table and the supporting substrate. Some of these interface configurations are intended to enhance the bond between the diamond table and the substrate, while others are intended to modify the types, concentrations and locations of stresses (compressive, tensile) resident in the diamond tables and substrates as a result of the cutter being formed in an ultra high-pressure, ultra high-temperature process. Such residual stresses, as known in the art, are prone to arise because the diamond table typically has a lower coefficient of thermal expansion than that of the substrate to which it is cojoined. Additionally, the diamond table and substrate will typically have differing values of bulk modulus, thereby compounding the likelihood of residual stress being present in the cutter. As a newly formed cutter cools from the elevated temperature required to form the cutter, the residual stresses in the cutter tend to be especially concentrated at and near the interface where the diamond or superabrasive table is disposed upon the supportive substrate. Thus, depending on cutter construction, the direction and magnitude of such residual stresses may, and often do, cause the diamond table or superabrasive layer to prematurely fracture, delaminate, and/or spall as compared to cutters in which residual stresses are fortuitously of lesser magnitude or in which the residual stresses by chance happen to be oriented favorably.
Many attempts have been made to provide PDC cutters which are resistant to premature failure. The use of an interfacial transition layer with material properties intermediate of those of the diamond and substrate is known within the art. The formation of cutters with noncontinuous grooves or recesses in the substrate filled with diamond is also practiced, as are cutter formations having concentric circular grooves or a spiral groove.
The patent literature reveals a variety of cutter designs in which the diamond/substrate interface is three-dimensional, i.e., the diamond layer and/or substrate have portions which protrude into the other member to xe2x80x9canchorxe2x80x9d it therein. The shape of these protrusions may be planar or arcuate, or combinations thereof
U.S. Pat. No. 5,351,772 to Smith shows various patterns of radially directed interfacial formations on the substrate surface, the formations projecting into the diamond surface.
As shown in U.S. Pat. No. 5,486,137 to Flood et al., the interfacial diamond surface has a pattern of unconnected radial members which project into the substrate, the thickness of the diamond layer decreasing toward the central axis of the cutter. U.S. Pat. No. 5,590,728 to Matthias et al. describes a variety of interface patterns in which a plurality of unconnected straight and arcuate ribs or small circular areas characterize the diamond/substrate interface.
U.S. Pat. No. 5,605,199 to Newton teaches the use of ridges at the interface which are parallel or radial, with an enlarged circle of diamond material at the periphery of the interface.
In U.S. Pat. No. 5,709,279 to Dennis, the diamond/substrate interface is shown to be a repeating sinusoidal surface about the axial center of the cutter.
U.S. Pat. No. 5,871,060 to Jensen et al., assigned to the assignee hereof shows cutter interfaces having various ovaloid or round projections. The interface surface is indicated to be regular or irregular and may include surface grooves formed during or following sintering. A cutter substrate is depicted having a rounded interface surface with a combination of radial and concentric circular grooves formed in the interface surface of the substrate.
Still other interface configurations are dictated by other objectives, such as particular, desired cutting face topographies. Additional interface configurations are employed in so-called cutter xe2x80x9cinsertsxe2x80x9d used on the rotatable cones of rock bits.
Other examples of a variety of interface configurations may be found, by way of example only, in U.S. Pat. Nos. 4,109,737, 4,858,707, 5,351,772, 5,460,233, 5,484,330, 5,486,137, 5,494,477, 5,499,688, 5,544,713, 5,605,199, 5,657,449, 5,706,906 and 5,711,702.
While cutting faces have been designed with features to accommodate and direct forces imposed on PDC cutters, see, for example, above-referenced U.S. Pat. No. 5,706,906, state-of-the-art PDC cutters have, to date, failed to adequately accommodate such forces at the diamond table-to-substrate interface, resulting in a susceptibility to spalling and fracture in that area. While the magnitude and direction of such forces might, at first impression, seem to be predictable and easily accommodated based upon cutter back rake and WOB, such is not the case, due to the variables encountered during a drilling operation previously noted herein. Therefore, it would be desirable to provide a PDC cutter having a table/substrate interface able to accommodate the wide swings in both magnitude and direction of forces encountered by PDC cutters during actual drilling operations, particularly in drilling formations of medium to high compressive strength rock, or containing stringers of such rock, while at the same time providing a superior, reliable mechanical connection between the diamond and substrate and sufficient diamond volume across the cutting face for extending the service life of the cutter, enabling more efficient and cost-effective drilling of boreholes in subterranean formations.
The present invention addresses the requirements stated above, and includes PDC cutters having an optimized table thickness and an enhanced diamond table-to-substrate interface, as well as drill bits so equipped.
The cutters of the present invention, while having demonstrated utility in the context of PDC cutters, encompass any cutters employing superabrasive material of other types, such as thermally stable PDC material and cubic boron nitride compacts. The inventive cutters may be said to comprise, in broad terms, cutters having a superabrasive table formed on and mounted to a supporting substrate. Again, while a cemented WC substrate may be usually employed, substrates employing other materials in addition to, or in lieu of, WC may be employed in the invention.
Cutters embodying the present invention comprise a superabrasive table formed of a volume of superabrasive material and exhibit a two-dimensional, circular cutting face mounted or cojoined to an end face of a generally cylindrical-shaped substrate. An interface between the end face of the substrate and the volume of superabrasive material includes at least one generally annular arcuate surface of substrate material which is defined, in cross-section taken across and parallel to the longitudinal axis of the cutter, by an arc and further includes at least one radially recessed portion, extending radially across the interface between the substrate and the superabrasive volume. The generally annular surface of the substrate preferably comprises a first spherical or spheroidal surface of revolution having a first radius of curvature and is generally centered about, or coincident with, the longitudinal axis or centerline of the cutter to form a convex surface generally in the center portion of the end face. The first spherical or convex surface of revolution is preferably radially adjacent and bounded at its periphery by another, second surface of revolution having a second radius of curvature. The second surface of revolution is preferably a portion of a toroid which provides a concave surface generally coincident to the longitudinal axis of the cutter and generally surrounds the periphery of the first spherical surface of revolution. Preferably, the concave surface is contiguous with the first spherical surface of revolution. The toroid, in which a portion thereof defines the concave surface, is defined by a second radius extending from a centerpoint radially offset from the longitudinal centerline. Radially adjacent and surrounding the periphery of the second surface of revolution is a third surface of revolution having a third radius of curvature. The third surface of revolution is preferably a portion of a second toroid which provides a radially outermost and uppermost convex surface with respect to the longitudinal centerline. The third surface of revolution is radially bounded by, and preferably contiguous with, a generally downwardly extending, radially inset annular side wall. The side wall may be generally planar and generally perpendicular with the longitudinal centerline or it may contain at least one annular chamfered portion preferably located longitudinally adjacent the third surface of revolution. The radially inset side wall intersects, and is preferably contiguous with, a circumferential rim, shoulder, or annular ledge and preferably is provided with radiused curvature at such intersection to minimize the possibility of localized stress concentrations arising thereabout. The annular ledge, shoulder, or circumferential rim is preferably generally perpendicular to the longitudinal centerline and extends radially outward to intersect a generally circular-shaped radially outermost side wall, as viewed from above, defining the radially outermost extent of the substrate.
In one embodiment, the radially extending recessed portion generally diametrically bisects a substantial portion of the interface surface by extending from one position of the radially outermost curved surface to another diametrically opposite position of the radially outermost curved surface and preferably terminates at the circumferential rim.
In another embodiment of the inventive cutter, the end face of the substrate includes a second recessed region or groove preferably bisecting the first recessed region or groove at the longitudinal centerline. The second groove is preferably oriented to be generally perpendicular to the first groove and is generally of the same configuration and dimensions.
In yet another embodiment of the inventive cutter, an end face having at least one larger recessed portion is provided with a plurality of circumferentially spaced smaller, second radially extending recessed portions. The plurality of second, smaller recessed portions preferably originate radially beyond the first surface of revolution and terminate short of the circumferential rim or annular ledge surface. Preferably, the plurality of second smaller recessed portions are of a lesser width and length than the at least one first larger recessed region.
A volume of superabrasive material is formed over the substrate end face in using high-temperature, high-pressure processes known within the art and preferably has a maximum thickness approaching or exceeding 0.160 of an inch with an initial minimum thickness of at least approximately 0.090 of an inch. However, other minimum and maximum table thicknesses can be used in accordance with the present invention. The volume of superabrasive material conforms thereto along the interface, including filling any recessed regions therein, and thereby forms a superabrasive table. The exterior surface of the table may be provided with features such as annular chamfers as are conventional and known in the art.
The surface of the substrate end face, by virtue of its generally arcuate cross-sectional configuration in combination with at least one traversely extending recessed portion, or alternatively, at least one traversely extending raised portion, provides an interface designed to address multi-directional resultant loading of the cutting edge at the periphery of the cutting face of the superabrasive table. In general, resultant loads at the cutting edge are directed at an angle with respect to the longitudinal axis or centerline of the cutter which varies between about 20xc2x0 and about 70xc2x0. The arcuate surface is designed so that a normal vector to the substrate material will lie parallel to, and opposing, the force vector loading the cutting edge of the cutter. Stated another way, since the angle of cutting edge loading varies widely, the arcuate surface presents a range of normal vectors to the resultant force vector loading the cutting edge so that at least one of the normal vectors will, at any given time and under any anticipated resultant loading angle, be parallel and in opposition to the loading. Thus, at the area of greatest stress experienced at the interface, the superabrasive material and adjacent substrate material will be in compression, and the interface surface will lie substantially transverse to the force vector, beneficially dispersing the associated stresses and avoiding any shear stresses.
Additionally, the at least one recessed region provided in the end face of the substrate, upon being filled with a superabrasive material, provides an enhanced heat transfer mechanism in which heat may be more efficiently conducted away from the cutting edge and the wear flat that typically forms on a portion of the radially outermost side wall and on a peripheral portion of the top face of the superabrasive table. Such an interface configuration, including a superabrasive material-filled recessed region or groove, tends to inhibit the formation of thermally induced cracks in the superabrasive table as well as the supporting substrate.